The invention relates to a system to be mounted onto aircraft doors, particularly in retrofit situations, but it could also be used for new aircraft assembly. Aircraft fuselages have a number of doors in addition to the primary doors that passengers enter and leave from, these additional doors being used by ground crew while the aircraft is on the ground. During commercial aviation operations, the crew has to work quickly to perform its various functions, accessing systems on the aircraft through these access doors. When the ground crew person is through with that task, he or she is to close the door to secure and latch it with a cam lock or thumb latch. While this is successful in most cases, occasionally the door is not closed completely, which can cause serious problems during flight. The open door not only can cause wind noise, but can fly off and fall to the ground, or cause damage to the aircraft. In either case, the aircraft must return to ground prematurely which can have its own negative consequences, particularly if it is landing with a full load of fuel, resulting in a hard landing that can necessitate further servicing, passengers missing connecting flight, aircraft being re-routed. Thus there is a need for a further system to allow a pilot or other user to make a final check that all of the doors are closed before takeoff.